The Roman Calendar has gone through several significant changes, and is now thought of as the Gregorian calendar, after the most recent calendar reform instigated by Pope Gregory in 1582. But let’s start at the beginning.
The early Roman calendar was a lunisolar calendar, which had months that were either 28, 29 or 31 days long, and years that were either 12 or 13 months long. The occasional “leap month” was added every few years to keep the civil calendar year synchronized with the seasonal year over the long term. One big problem with this method is that calendar years are either 354 days long or 384 days long; a big difference when it comes to book keeping and paying taxes!
Julius Caesar decided a calendar reformation was in order. The chaos of civil war in the late republic had disrupted the calendar and a few leap months had been missed, so the year 46 BC, three extra leap months were inserted to recalibrate the calendar. This year had 445 days and was nicknamed the “year of confusion.” The next year a new calendar system, the Julian Calendar, was adopted by the Roman empire, based on a new algorithm to keep the civil year in alignment with the seasonal year, which was determined to be on average 365.25 days long. Instead of adding a leap month approximately every third year, they added a leap day every 4th year. This meant that a civil year was either 365 or 366 days long. Seems like a great improvement, but there was one significant casualty: the lunar month.
The Julian Calendar no longer kept track of the lunar months. Instead they used 12 civil months that were fixed relative to the civil year. A Julian month lasts approximately the same length as a lunar month, but they are unrelated. The word ‘month’ is derived from the word moon, but the 12 so-called months of the Julian and later the Gregorian year have absolutely nothing to do with the Moon. Can you imagine how confusing this was to people when it was first introduced. You look at the calendar and it says a new month is beginning, but you look at the Moon and it may be a half Moon or a full Moon or any other type of Moon and you’ve gotta wonder what the heck is going on. But you don’t have time to figure it out, you’ve got taxes to pay.
The civil months of the Julian or Gregorian year are more closely related to the 12 signs of the zodiac. Most people hear about the signs of the zodiac and they think you are talking about astrology, but the 12 signs of the zodiac are really just 12 equal (30º) divisions of the ecliptic, which is a circular pathway through the background of the stars that the Sun appears to take as the Earth orbits the Sun over the course of a seasonal year. The Sun spends a little over 30 days in each sign, which once again is about the length of a civil month. Which begs the question, why not just use the 12 signs of the zodiac and give them the names of the 12 Roman months? The 12 signs of the zodiac are so arranged that one sign ends and another begins at each equinox and solstice. The Spring & Fall equinoxes and Winter & Summer solstices are extremely important astronomical or planetary events that mark the four corners of the ecliptic and the four corners of the seasonal year. In contrast, the Julian or Gregorian months are unrelated to the equinox or solstice events. The Gregorian months are approximately 20º out of phase with the signs of the zodiac. For whatever reasons, the Roman calendar months were not brought into alignment with the 12 signs of the zodiac. Maybe they figured: we’re disconnecting the lunar months from the civil months, we might as well stay consistent and keep the civil months disconnected from the equinoxes and solstices.
The Julian calendar algorithm lasted over 1500 years before the civil year got significantly out of alignment with the season year. But finally in 1582 Pope Gregory called for a calendar reformation to adjust the algorithm, where instead of a 4-year leap day cycle based on 365.25 days per seasonal year, the length of a seasonal year was postulated to be 365.2425 days, requiring a 400 year leap cycle that required slightly less leap days added every 4th year.
The excess leap days introduced by the Julian algorithm had caused the calendar to drift such that the spring equinox was occurring well before its nominal March 21 date. This date is important to Christian churches because it is involved in calculating the date of Easter. To realign the seasonal year with the civil calendar year, the reform advanced the date by 10 days: Thursday 4 October 1582 was followed by Friday 15 October 1582. The Gregorian algorithm has an error of about one day per 3,030 years. By the 19th century, the Gregorian calendar was adopted almost worldwide for the sake of convenience in international trade.